Study On Preparation Of Nanocrystalline Copper By Pulse Electrodeposition And Its Tribological Properties

Copper is widely used in the fields of transportation,machinery and electronics industry due to its excellent mechanical,electrical and thermal conductivity properties.However,the progress of technology requires higher and higher comprehensive properties of materials,so as to be able to adapt to harsh service environment.As for copper,its poor wear resistance is an important reason that restricts its wide application.In recent years,it has been found that the wear resistance of nanocrystalline copper is significantly improved compared with that of coarse-grained copper,and the wear characteristics and mechanism of nanocrystalline copper have been studied.However,as a novel material,the tribological understanding of nanocrystalline copper is not enough so far,so it is necessary to further understand the tribological properties of nanocrystalline copper in order to promote its practical application.In this thesis,nanocrystalline copper coatings and Mo S₂nanosheets/nanocrystalline copper composite coatings were prepared by pulse electrodeposition method,and their tribological properties were studied.The main research contents are as follows:1.The Nanocrystalline copper coating with an average grain size of 10nm was prepared by pulsed electrodeposition.Tribological studies were carried out using a ball-on-disc tribometer.The influences of GCr15 and Zr O₂counterpart on the tribological behaviors of nanocrystalline copper were investigated.SEM was used to observe the morphologies of the wear marks,wear debris and the ball counterparts,EDS was used to analyze the element composition of the wear debris and wear marks,Through these analyses,the wear mechanism of two friction pairs was explained.The results show that compared with Zr O₂/Cu pair,the friction coefficient of GCr15/Cu pair was higher with large fluctuation,and the wear mechanism was mainly abrasive wear,adhesive wear and fatigue wear,which leads to the higher wear rate of GCr15/Cu pair.The coefficient of friction of Zr O₂/Cu was lower and more stable,and the wear mechanism was mainly due to slight fatigue wear.Therefore,the wear rate of Zr O₂/Cu was lower.2.It is proposed that the wear resistance of nanocrystalline copper can be improved by using Mo S₂nanosheets to stabilize the friction interface and form a grain relaxed hardening layer.Mo S₂nanosheets were prepared by liquid phase ultrasonic stripping method.Four kinds of composite coatings with different contents of Mo S₂were prepared by using four kinds of composite electroplating baths containing 0mg/L,12.5mg/L,25mg/L and 50mg/L,respectively.The effects of different content of Mo S₂nanosheets in electroplating solution on the growth kinetics,structure,morphology,hardness,tribological properties and wear mechanism of electrodeposited composite coatings were studied.The results show that the Mo S₂nanosheets have an inhibitory effect on the deposition of the coating,and the high content of Mo S₂nanosheets will lead to the coating surface becomes rough,but the existence of Mo S₂nanosheets can make the deposited coating more compact.The results of friction and wear tests show that the Mo S₂nanosheet/nanocrystalline copper composite coating exhibits excellent tribological properties.The composite coating(12.5-Mo S₂/Cu)prepared from the electroplating solution with the content of 12.5mg/L Mo S₂nanosheets is the best.The friction coefficient(?0.19)of 12.5-Mo S₂/Cu under 1N load is significantly lower than that of the single nanocrystalline copper(?0.5)coating,and the wear rate is only 1/8 of that of the single nanocrystalline copper coating.The wear mechanism between GCr15 and nanocrystalline copper was changed from severe adhesion wear and fatigue wear to slight fatigue wear due to the presence of Mo S₂nanosheets.By measuring the hardness of the wear mark surface and observing the structure of the wear mark subsurface,It was found that a grain boundary relaxation hardening layer with a thickness of about 100nm was formed on the top of the wear mark.The formation of this hardening layer makes the friction layer become stable,which effectively improves the wear resistance of nanocrystalline copper.